1. Field of the Invention
The present invention relates to a method and apparatus for generating nano-bubbles in a liquid, and more particularly, to a method for generating nano-sized fine bubbles by using a bubble generating member made of naturally occurring materials without applying an external mechanical force to a liquid, and a nano-bubble generating apparatus suitable for implementing the method.
2. Description of the Related Art
Generally, gases newly generated in a liquid or permeated into the liquid create fine bubbles, are dissolved and absorbed in the liquid, or remain as bubbles in the liquid. The bubbles remaining in the liquid tend to have a smaller specific gravity in comparison with the liquid, so that the bubbles move upward and float. However, if the bubbles have a size smaller than a certain level, it is known that the bubbles do not float to the liquid surface but remain in the liquid as they are or shrink in the liquid by external pressure and are finally ruptured, so that the gases in the bubbles are dissolved in the liquid.
As mentioned above, in a case where specific fine bubbles are generated in a certain liquid, on the one hand, such a liquid dissolves specific gas, and on the other hand the liquid is present with the fine bubbles of the gas contained therein. At this time, in a case where the specific gas is selected to be useful for an organism and the certain liquid is also selected to be useful for or harmless to the organism, such a liquid material in this state may give a very useful effect to the organism. In addition, when the specific gas is selected to be useful for the industry, such a liquid material may be used as a very useful material for the industry. Thus, in the present invention, the liquid is preferably water harmless to the human body, or the liquid may be an aqueous solution obtained by applying a small amount of additive to the water, or other liquids.
Generally, in properties of bubbles dissolved in water, bubbles having a size greater than 50 μm float on the surface of water within several seconds and are diffused in the air, while bubbles having a size of 50 μm to some hundreds of nanometers may remain in the water up to 6 months. It is known that these bubbles gradually shrink and rupture in the water so that the corresponding gas is dissolved in the water over a saturated concentration.
When a large amount of bubbles with a fine size are generated at a time, the concentration of a gas dissolved in the water may be increased. When the bubbles remain in the water for a long time, the water may be advantageously utilized as a drinking water in which a gas such as oxygen or hydrogen is dissolved in a high concentration.
Conventional or applicable techniques for making a drinking water by generating nano-bubbles in water to increase the solubility of gas (oxygen) are known as follows.
A dissolved gas method is a method in which a closed container filled with water or beverage is charged with a gas at a high pressure, so that the gas is dissolved in the water or beverage. If this method is used, the gas dissolved in the water or beverage is released out as it bounds off the moment the closed container is opened. Using this property of gas, this method is utilized for charging a soft drink with carbonic acid gas. However, if this method is used, the gas dissolved in the water or beverage comes out of the container and is discharged out the moment the closed container is opened, and thus, the gas cannot be disadvantageously maintained in a state where it is dissolved in the water or beverage for a long time.
A hydrodynamic method is a mechanical mixing method, in which water and oxygen are ejected at the same time toward a motor propeller that rotates at a high speed of about 10,000 to 20,000 rpm so that oxygen gases are forcibly adsorbed to the surface of water particles. In general, a container is filled with a liquid such as water, and then, while applying a gas thereto from the outside, the gas and the liquid are mixed using a high-speed rotational force. However, in this method, the liquid material is rotated by using a high-speed rotational force, so that a fine physical-chemical structure of the liquid material is disadvantageously broken. In particular, in case of a drinking water, the water may suffer from stress during a manufacturing process. In addition, there is a disadvantage in that in this method a large amount of energy should be supplied in order to obtain a high-speed rotational force, and there is a limit in a dispersing force of the gas due to the high-speed rotation. Thus, if this method is used, there is a limit in decreasing a size of gas that is to be dissolved in a liquid material. Also, if this method is used, when the rotation of the liquid material is stopped, the size of gas dissolved in the liquid material cannot be decreased into a fine size, and thus, the gas dissolved in the water easily float and escape as time goes.
A water exposure method is a method in which water is sprayed to an upper space of an oxygen tank so that the water conies in contact with oxygen, and the water is repeatedly sprayed until the oxygen concentration reaches a desired level. However, this method does not allow easy production of the water with a high oxygen concentration in aspect of commercial utilization, and its productivity is too low to ensure applicability.
Meanwhile, a dissolved air floating method is a method in which air is firstly dissolved in the water under a high pressure to make supersaturated, air-dissolved water, and the air-dissolved water is re-injected into a water to be subjected to water treatment, such as sewage. In this method, fine air bubbles are generated from the air-dissolved water, and then the air bubbles are combined with the floc dissolved in the water to be subjected to water treatment, so that this method is used for purifying the water to be subjected to water treatment. However, since the dissolved air floating method uses the property of bubbles dissolved in water that tend to move upward, the bubbles should be formed to have a large size, that is the bubbles cannot be formed to be smaller than 50 μm.
There is also introduced a membrane method. In this method, a fine porous material is formed of ceramic or metal, and oxygen bubbles are formed through a porous membrane of a filter that is made of the fine porous material. However, this method has a technical limit since it is impossible until now to make a nano-sized fine porous membrane of ceramic or metal.
Prior art documents studied by the inventor until now are as follows: